A compound oxide represented by ABO.sub.3 (where A is a rare-earth element, part of which may be replaced by an alkaline earth metal, and B is one or more transition metal elements) offers a p-type semiconductor with perovskite types of crystal structure. It has been proposed that a semiconductor of this type is used as a sensitive material for a gas sensor, because the value of its resistance is reduced with, for example, an increase of the oxygen content in a gas.
U.S. Pat. No. 3,951,603 (Gas-sensor Element and Method for Detecting Reducing Gas or Oxygen Gas) discloses a gas sensor in which a paste including a perovskite type of compound oxide semi-conductor is coated and sintered onto an alumina substrate, and which is sensitive to a reducing gas.
U.S. Pat. No. 3,953,175 (Gas-sensor Element and Method for Detecting Oxidizable Gas) discloses a gas sensor which employs a sensitive material consisting of a mixture of perovskite types of compound oxides and K.sub.2 Mg.sub.4.
U.S. Pat. No. 4,044,601 (Smoke and Gas Sensor Element) discloses a gas sensor which employs a sensitive material consisting of a mixture of perovskite types of compound oxides and CdO, In.sub.2 O.sub.3, SnO, Tl.sub.2 O.sub.3 or PbO.
Further, a gas sensor using perovskite types of compound oxides as a sensitive material is also disclosed in U.S. Pat. No. 4,221,827, Japanese Patent Laid Open No. 132941/1980, No. 144391/1975, No. 8537/1981, No. 110385/1975, No. 166030/1980, No. 31631/1981, No. 35533/1981 and No. 166459/1981.
With such a gas sensor using perovskite types of compound oxide semiconductors as a sensitive material, it has been observed that some change occurs with the lapse of time, if the composition of, for example, a combustion exhaust gas, particularly the oxygen content, varies. More specifically, if a sensor is first exposed to perfect combustion flames and then to imperfect combustion flames, the value of the electric resistance of the sensitive material changes with time. When the sensor is returned into the perfect combustion flames once again, the value of its resistance is not restored to its original value instantaneously, but it returns gradually to it original value in equilibrium with the gas content over a certain period of time.
As shown in FIG. 1, as a result of combustion experiments using a gas sensor which employs a sensitive material consisting of only a p-type perovskite compound oxide (LaNiO.sub.3), it has been found that the value of the resistance of the sensor increases, when the conditions it is exposed to shift from a perfect combustion region a (where the flow rate of air is 10 l/min to 0.1 l of propane) to a region b where the flow rate of air is reduced to 9 l/min, in the presence of secondary air. When the flow rate of air is further reduced to 8 l/min, the resistance increases more (region c). After a reduction in the flow rate of air to 7 l/min (region d) and then 6 l/min (region e), even when the flow rate is increased gradually once again to 7 l/min (region f), 8 l/min (region g) and then 9 l/min (region h), the resistance does not return back to its original value and takes a somewhat higher value. The time period of each region in FIG. 1 corresponds to 1 minute.
It is desired that an ideal gas sensor offers the same value of resistance at all times under identical combustion conditions.
The above change with time in the perovskite types of compound oxide semiconductors probably results from a reversible change or conversion of the conductivity type of the compound oxide itself attendant on the adsorption and release of oxygen molecules as well as reducing substances (such as carbon monoxide), or from the influence of changes in temperature. In particular, chemical changes due to the adsorption and release of gas molecules are likely to appear with a time lag in the diffusion process of the gas molecules between the surface and the interior of the p-type perovskite compound oxide. This phenomenon (referred to as a tailing effect in this invention) can be also observed in n-type oxide semiconductors (such as tin oxide).
However, such a tailing effect is very disadvantageous in gas sensors used in fields where combustion conditions vary frequently, including combustion control based on the oxygen content or carbon monoxide content in the exhaust gas from motor vehicles as well as combustion controls of domestic heating appliances and boilers.